In an article in Scientific American magazine (vol. 290, no. 1, January 2004, incorporated by reference), Roy Want discusses RFID (radio-frequency identification) technology, which is widely used in retail theft prevention and vehicle registration at toll booths. Want states (page 58, bottom of third column), “higher-frequency [RFID] tags now enable a reader [detector] to identify many individual tags grouped together, even haphazardly—although they are not yet able to distinguish perfectly among all the items in a loaded grocery cart [which] is a major aim of this technology. Once perfected, such RFID scanning should steamline inventory and checkout procedures and save millions of dollars for retailers.” Want cites (page 65) The RFID Handbook by Klaus Finkenzeller, published by John Wiley & Sons, which is entirely incorporated herein by reference.
The probable reason that RFID cannot distinguish items jumbled haphazardly is that RFID tags are powered by radio signals from the RFID detector, which means that after they are exposed to the radio energy from the detector, they will respond after a certain interval. (The radio-wave energy they receive is accumulated in a capacitor until there is enough charge to run the tag-signal emitting circuit.) Therefore, if there are two cans of Grandma's Chicken Soup, 16 oz. size, in the grocery cart, the tags on both cans will respond at about the same time, if they are next to each other and similarly oriented. If they are not oriented the same, or are at different distances from the detector, or are more or less obscured by a radio-blocking object (such as a metallic food package), then they will respond at different times.
In any event, there will be no information available from the timing of the identifying signals from the two tags of the two cans.
Also, conventional tags do not have a definite repetition rate at which they send out identification information, so if the tags' information is sent out repeatedly in identifiable bursts, then the repetition interval of any such bursts is not constant or steady.
It appears that, because there is little information available from the timing of the signals from the RFID tags, there is little possibility of using that timing to count the number of one item (e.g., Grandma's Chicken Soup, 16 oz. size) in the cart.